This is an investigator-initiated, partially industry-supported, two-center, randomized, placebo controlled trial of the a-glucosidase inhibitor acarbose(Precose, Bayer Pharmaceuticals, Inc.) in adults with early postprandial diabetes mellitus. Subjects will be followed up to 5 years for the development of overt diabetes mellitus with fasting hyperglycemia. They will also have evaluations for diabetes complications. Half(50%) of the volunteers will undergo a yearly evaluation to assess insulin secretion and sensitivity using a hyperglycemic clamp study.